This invention relates to lead frames for the manufacture of semiconductor devices encapsulated in a plastic material and more particularly to lead frame structures which can be manufactured with greater versatility and a reduced loss of material thereby significantly reducing the cost of manufacture.
The development of more highly automated techniques for the manufacture of semiconductor devices has resulted in the development of various types of lead frames by which the relatively small and fragile metallic device leads ultimately associated with a semiconductor device are formed as a multiple unit preassembled structure which is structured to maintain mechanical rigidity by the use of tie bars and which is adapted for automatic assembly techniques with index holes. These mutliple unit lead frames allow automatically indexed sequential operations and batch handling and result in reduced manufacturing costs. Initially these lead frame approaches were proposed for low power transistor structures which were relatively small as in Helda et al, U.S. Pat. No. 3,444,441. Later developments expanded these techniques to physically larger transistor structures having much higher power dissipation requirements as in Segerson, U.S. Pat. No. 3,574,815. When semiconductor structures having higher power dissipation requirements are manufactured, an additional constraint is introduced in that the portion of the lead frame structure used to provide physical support for the semiconductor device must now be adapted to not only physically hold the semiconductor device but to effectively conduct heat away from the semiconductor device to avoid the build-up of high temperatures within the semiconductor device junctions which tend to destroy the device or reduce its long term reliability. Typically the method employed to improve the heat dissipation characteristics of this portion of the semiconductor device lead frame is to make the device support portion physically larger or thicker and to use metallic material particularly adapted to improve heat conduction. Thus the optimum characteristic of the die support portion of the semiconductor device lead structure may not be optimum for providing the other requirements of the semiconductor device leads. One prior art method used to overcome this problem is to reduce the thickness of the lead portion of a semiconductor lead frame by machining or by stamping while retaining the original thickness for the portion of the lead frame used for semiconductor device support. The desire to simply assembly procedures and to reduce the cost of materials used in fabricating semiconductor device lead frames also indicates that many advantages would accrue from higher density semiconductor device lead frames.